Phenol-formaldehyde resins are widely used as adhesives and binders in the making of laminated products such as plywood.
One process for applying phenol-formaldehyde resins to wood laminates such as plywood is the foam extrusion process. Historically the use of the foam extrusion process to apply solutions of phenol-formaldehyde resin presupposed, of course, that the resin solutions were foamable. At first, it was possible to foam such resin solutions adequately since relatively low molecular weight phenolic resins were used in their manufacture. However, the use of phenolic resins of low molecular weight creates an economic problem in that a relatively long press time is required to cure them in the plywood hot presses. This in turn delays production and increased costs.
In an effort to overcome this problem, the plywood industry has turned to the use of more highly advanced (higher molecular weight) resins which cure faster in the press. Presently the highly advanced resins are supplied to the market in aqueous alkaline solutions containing about 40% by weight resin solids, as opposed to the low molecular weight solutions containing about 50% resin solids that were formerly used extensively.
A serious problem attended the use of highly advanced phenolic resin glues using conventional foaming techniques because they are not foamable at all, or at least with sufficient speed to be useful in the commercial production of plywood using foamed glue.
The serious problem of foaming highly advanced phenolic resin glues was overcome by the addition of various ingredients including a foaming agent such as blood.
At the present time, when phenolic glues are applied to wood laminates using the foam extrusion process, the solution of phenolic resin is mixed with water, glyoxal, spray dried blood, wheat flour, sodium hydroxide and surfactant, or various other ingredients, and then pumped through a mechanical foamer where air is introduced. The resulting foam which has a specific gravity 0.2 or less is then extruded onto the veneer sheets which are consolidated to form panels. The panels are then hot pressed to form a strong exterior bond. In this application, the terms veneer and veneer sheet are used to mean any of the thin layers or plies of wood which are bonded together to form plywood.
One key ingredient in foaming solutions of advanced phenolic resins is spray dried blood which acts as a foaming agent and also enhances foam stability. Even though blood contributes much to the phenolic based glue, it is very expensive compared to the other ingredients. Although blood is a good foaming agent, frequently the foam density still varies resulting in skipped spread and poor bonds, which if severe enough can reduce plywood production.
Plywood consists of veneer sheets bonded together with grains of alternate layers perpendicular to each other. Veneer is usually made by the rotary lathe method which comprises rotating a log of wood against a knife. Once the rotary peeled veneer is clipped, dried, and graded, it is ready to be made into plywood. The veneer sheets are spread with adhesive, and then consolidated into a package that is conveyed into a prepress that applies 125-175 psi for about three minutes. After prepressing, the package is conveyed to the hot press. Here the panels are fed into a multiple opening steam press and pressed to cure the phenolic resin. A press temperature of 285.degree. F.-360.degree. F. and pressure of 125-200 psi is used. Press time is dictated by panel thickness and particular lay-up. Assembly time ranges anywhere from 10 to 40 minutes with 15-20 minutes being most common. After hot pressing, panels are usually allowed to post cure in the stack for at least two hours before sawing. However, some mills immediately convey them to the saw line where they are trimmed to size.
When adhesive is applied to the veneer, several methods can be used such as rubber roll spreading, airless spraying, and curtain coating. However, foam extrusion adhesive application has come into use.
Phenolic resin based adhesives are well known in plywood manufacture. As originally adopted, phenolic adhesives were applied between confronting plies in liquid or droplet form. With the advent of a process for extrusion of foamed glue, phenolic resin based adhesives have been adapted for foam extrusion application.
Phenolic adhesives are typically foamed and then applied by a foam extruder. Foaming usually increases the volume of the adhesive about five times. The foam is extruded in flexible coherent parallel rods on the surface of veneer to be bonded, as it travels under an extrusion head.
In the preparation of foamed adhesive, as a first step, unfoamed adhesive travels through a defoamer where it is brought to a uniform density of about 0.93 grams/cc. After defoaming, the adhesive may be stored in a feed tank until it is to be foamed. From the feed tank it is metered into a foamer which rapidly converts unfoamed adhesive to foamed adhesive. The foamed adhesive is then directed through the extrusion head onto the surface to be bonded.
After extrusion of the rods of foamed adhesive onto the top surface of a veneer sheet, a second layer is placed over the adhesive and the layers are roller pre-pressed. Pre-pressing spreads the rods of foamed adhesive into a uniform mat which wets the adjacent surface and insures good contact between the layers.
In the foam extrusion process, if the phenolic resin base is a highly advanced resin, a foaming agent such as dried blood must be added to the resin to improve its foamability.
The Cone et al patent, U.S. Pat. No. 3,705,921, describes a plywood glue in which blood is used to improve the foamability of a more highly advanced phenolic resin. The advanced phenolic resin used by Cone et al was a resin of commerce produced by Borden, Inc., the assignee of this invention.
For the purposes of this invention, a solution of highly advanced phenolic resin solution is defined as a solution which is the condensation product of a phenol, an aldehyde, water and alkali and which has a resin solids content of from 35% to 45% by weight of resinous condensation product and a degree of advancement such as to impart to the aqueous resin solution a viscosity of from 300 cps to 1500 cps as measured on a Brookfield RVF viscometer at 25.degree. C. with a #2 spindle at 20 RPM and has an alkali content of from 5.5% to 7.6%.
The term alkali as used herein, is defined as any of the hydroxides or soluble salts of alkali metals. The term alkalinity of a resin solution as used herein is defined as a measure of the basic properties provided by alkali compounds and alkaline earth compounds in the resin solution. The term "alkali content" of a resin solution as used herein is defined as a calculated figure of the amount of anhydrous sodium hydroxide by weight, based on the total weight of the resin solution, that would provide the same degree of alkalinity that is provided by the alkali and alkali earth compounds that are actually in the resin solution.
One of the major problems that occurs with foam extrusion glue application is that intermittently small portions of the glue will not foam. When this happens, glue foam density increases and extrusion beads break, causing areas of the ply surface to be void of glue. As one would expect, this produces poor quality plywood.
Poor glue foamability is due to a variety of factors that are very subtle, hard to define and control. The phenolic resins made according to this invention inherently foam better than comparable phenolic resins of the prior art and use of the resins of this invention should overcome some of these subtle problems.